This invention relates to the field of electro-optical devices, and more particularly to a method for manufacturing high efficiency light emitting diodes (LED).
Most high illumination, high efficiency light emitting diodes (LED) in use today are AlGaAs infra emitting diodes (IRED), red LEDs, AlGaInP red, orange, yellow, green-yellow LEDs, or AlGaInN blue and green LEDs. AlGaAs and AlGaInP LEDs are fabricated on a GaAs substrate, which absorbs more than half of the light emitted from the emitting layer. This is considered energy inefficient for most practical applications. To improve the power efficiency, most AlGaAs-based LEDs grow a thick AlGaAs epitaxial layer with energy bandgap larger than the light-emitting layer, as discloed in U.S. Pat. No. 5,185,288. The epitaxial layer is used as a transparent substrate, and the light-absorbing GaAs substrate is removed. However, in this method, it takes a much longer time to grow the AlGaAs transparent substrate, and hence higher fabrication cost. Furthermore, because the crystal grid and heat expansion coefficients of the AlGaAs and GaAs arc different, the grown epitaxial layer usually suffers sever warpage and deformation. Consequently, the following fabrication of LEDs is more difficult. In addition, the AlGaAs substrate, as similar to the GaAs substrata, is not good in heat dissipation and, hence, unsuitable for high current opertion.
The AlGaInP LEDs, as disclosed in U.S. Pat. No. 5,376,580, use the method of direct wafer bonding to form the transparent substrate. However, the method removes the GaAs substrate before the bonding the AlGaInP epitaxial layer to another transparent GaP substrate. Because the AlGaInP epitaxial layer, after removing the GaAs substrate, is only about 50 um in thickness, it is easier to break during the wafer bonding process and, hence, the yield rate is decreased. In addition, the heat dissipation in a GaP substrate, although slightly better than in a GaAs substrate, is still far below metals, such as Al or Cu. Therefore, the LEDs manufactured with this method can not operate at a high current.
U.S. Pat. No. 6,319,778 B1 discloses another method of manufacturing LEDs. It uses a metal with a high conductivity and a high reflection coefficient to avoid the light absorption by the substrate. Because the bonding process is conducted at A low temperature, the solder layer can be fused into the liquid state to achieve a better soldering. In addition, the industrial standard requires the vertical structured LEDs to provide a single wire bonding to lower the packaging cost. Because of the better heat dissipation capability, the LEDs manufactured with this method can operate at a higher current.
U.S. Pat. No. 2001/0,042,866 A1 discloses method of metal bonding for manufacturing the high illumination AlGaInN LEDs. It uses a metal reflection layer to reflect the emitted light, to avoid the light absorption by the substrate. The metal bonding layer bonds the epitaxial layer to a substrate with a good heat dissipation capability, such as Si, and a metal substrate to improve the heat dissipation. However, for a good yield rate with this method, both the surfaces of the heat dissipation substrate and the LED epitaxial layer must be smooth before the bonding. As the LED epitaxial layer is usually warped, and contains small bumps at the surface, it is difficult to achieve a good bonding between the two.
The present invention solves the problem of light absorption by the substrate, improves the heat dissipation, and can transport the LED epitaxial layer from the poor heat dissipation substrate to the good heat dissipation metal layer even when the LED epitaxial layer is warped or has a rough surface.
The present invention is related to a method of manufacturing high efficiency LEDs. The LED uses a metal reflection layer to solve the problem of light absorption by the substrate, and improves the illumination. To avoid the interaction between the metal reflection layer and the upper cladding layer of the LED, a diffusion barrier layer is used between them to prevent their interaction. It also forms a vertical structure where the P and N ends are on the top and bottom sides of the LEDs, respectively. A vertical structure is easier for final packaging. In addition, the present invention uses a metal substrate to replace the semiconductor substrate in order to improve the heat dissipation, and enable the LEDs to operate at a higher current.